An article by Dr. techn. H.-P. Grasal in "Elektronik" 6/22.03.1985, pages 61 et seq., discloses a surface wave convolver which is used as a signal-processing component. The surface wave convolver described therein consists of an elongate piezoelectric substrate, to one end of which a first interdigital transducer is fitted, one connection of which serves as signal input for electrical input signals to be processed. These input signals are converted by the interdigital transducer into acoustic surface waves which propagate along a propagation surface defined by an integration electrode on the piezoelectric substrate. At the other end of the piezoelectric substrate there is disposed a further interdigital transducer, one connection of which serves as a signal input for a predeterminable electrical signal which is converted, in a similar manner, into acoustic surface waves which propagate on the piezoelectric substrate in the direction of the first interdigital transducer. In this manner, the surface wave convolver executes a physical folding of the electrical signals which are present at its signal inputs. The signal output of the convolver is formed by a point of connection of the integration electrode. At this signal output it is possible to pick off a convolver output signal, which is proportional to the convolution integral of the two input signals, as long as those signal components of the input signals which contribute to the result of the convolution are found, in a condition entirely converted into surface waves, under the integration electrode of the surface wave convolver. The integration period of the surface wave convolver is determined by the ratio of the length of the integration electrode to the propagation velocity of the surface waves on the substrate.
An article by H.-P. Grassl and H. Engan ("Small-Aperture Focusing Chirp Transducers vs. Diffraction-Compensated Beam Compressors in Elastic SAW Convolvers" in "IEEE Transactions on Sonics and Ultrasonics", Vol. Su-32, No. 5, Sep. 1985) discloses that real surface wave convolvers exhibit a nonuniformity. This is due, among other reasons, to the fact that the signal components forming the convolver output signal originate from arbitrary locations under the integration electrode. In order to permit an approximately equal amplitude weighting and phase retardation for all these signal components on their path to the signal output, the integration electrode is designed as a gathering and matching grid having a plurality of pick-off points uniformly distributed over its entire length. A variation of the electrical parameters in the grid structure of the integration electrode, a variation of the series resistance of the propagation surface and, especially, standing-wave effects of nonlinearly generated longitudinal waves where the substrate thicknesses are not constant lead to a nonuniformity of the surface wave convolver. Accordingly, the uniformity is interpretable as a weighting function of the integration electrode. In operation of the surface wave convolver, i.e., when the input signals to be processed are applied to one of its signal inputs and a predeterminable signal, which corresponds to the desired pulse response of the surface wave convolver (programmable filter), is applied to its other signal input, the nonuniformity leads to a situation in which that output signal of the surface wave convolver which represents the result of convolution of the input signals is affected by a considerable error. Even where the greatest care is taken in the production of the surface wave convolver, the abovementioned causes of the nonuniformity cannot be entirely eliminated.